Many paper documents are promptly discarded after being read. Although paper is inexpensive, the quantity of discarded paper documents is enormous and the disposal of these discarded paper documents raises significant cost and environmental issues. Accordingly, there is a continuing desire for providing a new medium for containing the desired image, and methods for preparing and using such a medium. In aspects thereof it would be desirable to be reusable, to abate the cost and environmental issues, and desirably also is flexible and paper-like to provide a medium that is customarily acceptable to end-users and easy to use and store.
Although there are available technologies for transient image formation and storage, they generally provide less than desirable results for most applications as a paper substitute. For example, alternative technologies include liquid crystal displays, electrophoretics, and gyricon image media. However, these alternative technologies may not in a number of instances provide a document that has the appearance and feel of traditional paper, while providing the desired reimageability.
Imaging techniques employing photochromic materials, that is materials which undergo reversible or irreversible photoinduced color changes are known, for example, U.S. Pat. No. 3,961,948 discloses an imaging method based upon visible light induced changes in a photochromic imaging layer containing a dispersion of at least one photochromic material in an organic film forming binder.
One type of composition that can be used for forming photochromic papers is disclosed in Buncel et al. (J. T. C. Wojtyk, P. M. Kazmaier, E. Buncel, J. Chem. Soc. Chem. Comm, 1703, (1998)). The composition exhibits life-times of at least two days for solutions in acetone of spiropyrans modified with chelating groups in the presence of metallic cations. The metal cation Mn+ can stabilize the open merocyanine form through chelation.
These and other photochromic (or reimageable or electric) papers are desirable because they can provide imaging media that can be reused many times, to transiently store images and documents. For example, applications for photochromic based media include reimageable documents such as, for example, electronic paper documents. Reimageable documents allow information to be kept for as long as the user wants, then the information can be erased or the reimageable document can be re-imaged using an imaging system with different information.
Although the above-described approaches have provided reimageable transient documents, there is a desire for reimageable paper designs that provide longer image life-times, and more desirable paper-like appearance and feel. For example, while the known approaches for photochromic paper provide transient visible images, the visible images are very susceptible to UV light, such as is present in both ambient interior light and more especially in sun light. Due to the presence of this UV light, the visible images are susceptible to degradation by the UV light, causing the unimaged areas to darken and thereby decrease the contrast between the desired image and the background or unimaged areas.
That is, a problem associated with transient documents is the sensitivity of the unimaged areas to ambient UV-VIS light (such as <420 nm) where the photochromic molecule absorbs. Unimaged areas become colored after a period of time, decreasing the visual quality of the document, because the contrast between white and colored state is reduced. One approach, described in the above-referenced U.S. patent application Ser. No. 10/834,529, is to stabilize the image against light of wavelength<420 nm by creating a band-pass window for the incident light capable of isomerising (i.e. inducing coloration) in the material, centered around 365 nm. However, the unimaged areas of the documents still are sensitive to UV-VIS light of wavelength centered around 365 nm.